MAINTAINERS: unify reference to xen-devel list

In the linux kernel MAINTAINERS file, largely
  "xen-devel@lists.xenproject.org (moderated for non-subscribers)"
is used to refer to the xen-devel mailing list.

The DRM DRIVERS FOR XEN section entry mentions
xen-devel@lists.xen.org instead, but that is just the same
mailing list as the mailing list above.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Juergen Gross <jgross@suse.com>